Voice QOS Design Notes:

with LLQ the entrance criteria to a class can be as granular as you like because you define it by an ACL. You are not limited, as with IP RTP priority, to a simple UDP port range. If the port range feature did not get changed

Voice QOS Design Considerations Notes:

General Considerations

  • Do not use VoIP on a FR PVC that also carries VoFR
  • Prioritize the PVC if it carries only voice traffic
  • Don’t mark voice packets as DE
  • Set IP Precedence = 5 on the dial peer
  • Don’t use WRED for voice queues
  • Turn on DTMF-relay for low bit-rate codecs (8k and below)
  • Set echo, loss/gain parameters as specified by the network loss plan.
  • Measure/calculate network packet delay – the goal is 150 to 200ms one-way.
  • If TCP delays affect DTMF-relay performance use Cisco-rtp for DTMF-relay

Queuing Considerations

  • LLQ – classify voice in a priority class
  • Use ip rtp priority if LLQ is not available
  • Set the bandwidth on the priority statement in the LLQ configuration (or in the ip rtp priority statement) to the aggregate number of calls per interface/PVC
  • Create ACLs that prioritize both voice media and signaling

Fragmentation Considerations (speeds less than 1.5Mbps

  • Fragment to a 10ms delay to optimize size for backbone packet/cell sizes and network delay characteristics
  • Set fragments size so that voice packets are not fragmented
  • Set the ppp multilink fragment-delay command on leased line interfaces
  • Set the frame-relay fragment command in the FR map class
  • Fragment all PVCs carrying data on the interface if at least one PVC carries voice

Traffic Shaping Considerations

  • Set Be to 0
  • Set Bc to 10ms (1/100 of CIR) for mixed voice/data PVCs
  • Set mincir greater than or equal to bandwidth needed for voice
  • Set FRTS on the interface
  • Shape traffic strictly to the CIR on the PVC carrying voice
  • Shape both sides of the VC to the slower link speed to prevent egress blocking

CAC Considerations

  • Limit voice calls to prevent oversubscription of the bandwidth

Video QoS Design Considerations

  • For bidirectional and/or low speed video use priority queuing and allocate 384Kbps for bandwidth
  • For one-way video traffic use a CBWFQ mechanism

LLQ Considerations

  • Large video MTUs placed in the LLQ’s priority queue with voice traffic will bypass the fragmentation engine and cause delays for the voice traffic.

CAC Considerations

  • In a single-zone WAN model limit the number of video terminals
  • In a multizone WAN model use Gatekeeper CAC. However, Gatekeeper CAC is only available in a hub-and-spoke network.

A Networker Blog

2 thoughts on “Voice QOS Design Notes:

  1. hello,
    I have an ideea for the “# For bidirectional and/or low speed video use priority queuing and allocate 384Kbps for bandwidth” section:
    I think Cisco recommends for an interactive video class an extra 20% on top of the actual bandwidth requirements of the stream due to the bursty nature of this kind of traffic.

  2. Hi Alex, thanks for your comment!!

    A typical voice call will require 17 to 106 kbps of guaranteed priority bandwidth plus an additional 150 bps per call for voice-control traffic. Multiplying these bandwidth requirements times the maximum number of calls expected during the busiest time period will provide an indication of the overall bandwidth required for voice traffic.

    Now, Videoconferencing applications also have QoS requirements very similar to voice. But videoconferencing traffic is often bursty and greedy. The minimum bandwidth for a videoconferencing stream would require the actual bandwidth of the stream (dependent upon the type of videoconferencing codec being used) plus some overhead (yes 20% by Cisco Recommendation). For example, a 384-kbps video stream would actually require a total of 460 kbps of priority bandwidth.

    Best Regards

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